Impact of tropical American invasive species on ecosystem degradation in Himachal Pradesh – Northwestern Indian Himalayas

Himachal Pradesh, an Indian state in Northwestern Shivalik range of Himalayas, is known for its rich flora and fauna. However, during the last few decades, rich plant diversity of the state is under tremendous pressure from invasive plants either introduced or entered accidentally. These form their own monocultures in the invaded sites, degrade habitats, deplete biodiversity and thus alter structure and composition of natural communities. The establishment of invasive plants in various parts of the state may be attributed to multiple reasons like favourable environment, availability of open or degraded sites and human interference. A survey of the state conducted during the years 2002–2004 using quadrat method revealed the presence of 222 exotic invasive plant species. Out of these, 63 were from the South and Central America with Parthenium hysterophorus, Ageratum conyzoides, and Lantana camara as the most predominant ones comprising nearly 32% of the total plant density. Ageratum conyzoides had the maximum IVI (Importance Value Index) of 10.49% compared to 6.59% of P. hysterophorus and 7.52% of L. camara, respectively. However, percent cover area of L. camara was the maximum (~17.80%) compared to other two weeds. These three invasive weeds have displaced native flora and reduced their abundance thus threatening the integrity of native ecosystems. Maximum percent loss of species was in the L. camara invaded sites, where there was nearly 43% species loss compared to 36% in P. hysterophorus and 30% in A. conyzoides, respectively. In addition to competition, allelopathy (chemical interference) was determined to be one of the major strategies for niche exploitation by these exotic species in the alien invaded environments. The paper discusses the impact of invasion by these species on the structure, composition and dynamics of the natural ecosystems.